Our previous work in rats suggests that cortical neurons projecting to brainstem premotor cell groups and spinal cord via the pyramidal tract (PT-type) preferentially target striatal neurons projecting to the external pallidal segment (GPe), while cortical neurons having only intratelencephalic projections (IT- type) preferentially target striatal neurons projecting to the internal pallidal segment (GPi) and/or the substantia nigra pars reticulata (SNr). These findings suggest that PT-type corticostriatal neurons may provide striato-GPe neurons with information about cortical motor commands needed for their role in suppressing potentially conflicting movements, while integration of IT-type input from diverse cortical areas may be required for striato-GPi/SNr neurons to play their role in initiating desired movement. Synaptic facilitation or disfacilitation of subsets of these inputs could play a role in motor learning. Our conclusions about differential cortical inputs to the two main types of striatal projection neurons are, however, based on preferential but not exclusive labeling of striatal neuron types. Moreover, we did not distinguish between striato-GPi and striato-SNr neurons in these prior studies. Thus, the extent to which each of the three main types of striatal projection neurons in rats receive input from more than one type of cortical neuron remains uncertain. Additionally, we also do now know if our findings for rats are true for primates, and thus clinically relevant to the human basal ganglia. In Aim 1 of the current proposal, we will use in vivo intracellular methods in rats to record from individual striatal projection neurons and then at the end of the physiology session identify their type by biocytin-filling the neuron (and later tracing the axon of each to its destination). For each neuron we will use electrophysiological and LM/EM anatomical methods, so as to characterize the extent of the specificity of the IT input for striato-GPi/SNr neurons and the PT input for striato-GPe neurons. In Aims 2 and 3, we will determine by dextran amine labeling, immunolabeling and EM analysis if IT-type terminals preferentially target striato-GPi and striato-SNr neurons while PT-type terminals preferentially target striato-GPe neurons in monkeys. Given the critical roles of the cortical input to striatum in providing an instructive signal to the striatum and in the plasticity underlying motor learning, our studies will: 1) help reveal how the striato-GPi/SNr and striato-GPe neurons play complementary roles in motor control; 2) help clarify the mechanisms underlying the role of the basal ganglia in movement initiation and in the execution of movement sequences; and 3) help explain the relationship between the role of the basal ganglia in motor learning and in motor performance. PUBLIC HEALTH RELEVANCE This study will clarify which neurons of cerebral cortex communicate with each of the two circuits of the basal ganglia, one of which facilitates desired movements and the other suppresses unwanted movements. The findings will clarify how the information provided by cerebral cortex enables the basal ganglia to play its role in movement control and in the learning of new motor routines. The findings will suggest new insights into the role of abnormalities in the cortical input to striatum in Huntington's disease, Parkinson's disease, Tourette Syndrome, and obsessive-compulsive disorder, and thereby suggest new therapeutic approaches for treating these disorders.